How to Strengthen Floor Joists From Beneath

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how to strengthen floor joist from beneath

In many homes, it is common for floors to start to sag after a while. 

This common occurrence usually is not something to be worried about. However, if the floors are sagging too much, it could be a problem. 

Most of the time, when floors are sagging, it is because the joists have become worn out. As the wood ages, it will lose some of its strength and will bend when you walk on it. 

Fortunately, there are ways to fix this. By strengthening the joists from beneath, you can make sure your floors are safe and secure

In this article, we have laid out a few ways to strengthen floor joists. By choosing the one that will work best for you, you can be certain your floors won’t fail. 

Strengthening Floor Joists From Beneath: Basic Idea 

There are a few different methods available for strengthening floor joists from beneath. These include: 

  • Reinforce with plywood 
  • Apply blocking 
  • Use steel 
  • Add a joist beam 

Why Strengthen Floor Joists? 

why strengthen floor joist

If you have a basement or crawlspace under your home, you may experience floor sagging. When this happens, the floor can feel unsteady or less secure than it should. 

If you have a basement, a sagging floor means that the joists holding it up are weakening. This could potentially lead to someone falling through the floor. 

If you want to make sure that your home is safe and secure, strengthening the joists is a good option. 

In many municipalities, it is against building codes to have sagging floors. 

A building inspector will want to make sure that any home is safe for all kinds of people. If they find out your joists are weak, you could end up getting a fine. 

To ensure that no one gets injured at your home, it is best to make your floor as secure as possible. 

Add Plywood Reinforcements

One of the simplest ways of strengthening floor joists is reinforcing them with plywood. 

¾” plywood is the best material to use for this method and will ensure a strong bond. It is also widely available and relatively inexpensive compared to other methods. 

In most homes, there will be a top and bottom beam holding up the floor. Rip the ¾” plywood to the same width of those joists. 

Then, lift the joists up to the height you need them to be. This can be done with a jack or cinder blocks. 

Glue and nail the pieces of plywood between the two sections using 10d nails. Do the same thing on the other side of the beam using another piece of plywood. 

Use Blocking 

use blocking

The most effective way to strengthen your floor joists is to use the blocking method. 

Most of the time, when the joists are sagging, this is because only one of them has become weakened. By blocking one to the next, you can reinforce it. 

To do this, you will need a piece of lumber that is the same thickness as the joists. 

The lumber will be installed perpendicularly between the two joists. You will need enough lumber for one row of blocks for every 12 feet of joists. 

Using 10d nails, install the blocks between the joists in a straight line along the floor. This will provide enough reinforcement to keep the floor from sagging. 

Strengthen Them With Steel 

If you want a long-lasting solution, you can strengthen your joists using a steel plate. 

You can either attach the plate to the joist lengthwise to strengthen it or across it. This will depend on how the joist is bending. 

It is recommended that you use ½” or ¾” steel for this method. This will give you the best strength possible and will last for many years. 

One thing to consider with steel plates is that you can not drill through them. If you need to install any wiring through the joists, you will need to route it around the plate. 

Add a Beam 

Much in the same way as using the plate, you can simply add another beam to the one that is sagging. 

This is a great way to add some strength and still be able to drill through it for wiring. This method is called “sistering” the joists. 

To do this, you will need to use ⅝” carriage bolts and attach the new joists to the side of the old one. 

Make sure that the new joists are cut to the same size and that they have the same thickness measurements. This will ensure that everything stays put and that your floor will not start sagging again. 

Frequently Asked Questions 

faq how to strengthen floor joist from beneath

How much floor sag is acceptable?

According to most building codes, your floors can only sag less than one inch before they are out of compliance. 

While one inch may not seem like much, it will be very noticeable on the floor. If you are noticing any kind of dip in your floors, they could be in violation of the code. 

Are sloping floors in older homes normal?

While it is not uncommon for floors to slope in older homes, it should still be fixed. 

Sloping floors are an indication that the joists are starting to weaken. This could end up getting worse or could be a sign of a major structural issue. 

Can a house settle after 20 years?

Houses will settle over a number of years no matter how well they are built. 

For the most part, if a house is settling a few inches, this is an indicator that the soil has eroded. If the home is structurally sound, this shouldn’t present any issues. 


If your floor joists need to be strengthened, it is important that you do it as soon as possible. 

By using the above methods, you can make sure that your home is safe and structurally sound. You can also be certain that you won’t get any fines from the building inspector. 

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Michael J. O’Connor is a writer and marketing specialist from the Bay Area of California. A graduate of Sonoma State’s Creative Writing program, he spent many years as a contractor and carpet layer, learning the ins and outs of flooring and general contracting. When he’s not typing away at his desk, he enjoys hiking with his dogs, woodworking and collecting rare books. See full biography here.

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